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New submitter Chris Greenley writes "The T2K long baseline neutrino experiment in Japan has just announced conclusive evidence for electron to muon neutrino oscillation at the 7.5 sigma level. (The level needed for discovery is 5 sigma.) This experiment generates a focused beam of electron neutrinos using an accelerator in the J-PARC facility north of Tokyo which is aimed at the massive Super-Kamiokande detector 295 km (185 miles) away, near the west coast of Japan. 'This T2K observation is the first of its kind in that an explicit appearance of a unique flavor of neutrino at a detection point is unequivocally observed from a different flavor of neutrino at its production point.' This result clears the way for CP-violation neutrino studies which could show that 'regular' neutrinos act differently than their antimatter counterparts, a phenomenon that so far has only been observed in quarks. If neutrino CP-violation is found, it could explain why there is such a large predominance of matter over antimatter in the universe."

They detected 28 electron neutrino interactions, where they would have expected 5 such events without the oscillation in question. This helps underscore how incredibly hard is it to get neutrinos to show up with anything: even when one is manufacturing millions of them, one is lucky to get a tiny set to then show up in your detector. This is connected to how most neutrino detectors are basically large vats of water or some other liquid, because the most we can generally hope for is that if we put enough mass in the way, some neutrinos will by sheer chance run into things.

This is also relevant to what we expect for stellar neutrino observation. Understanding neutrino oscillation gets us a better idea of what sort of neutrino ratios to expect (as a function of energy levels) in other circumstances. Right now, we can observe a lot of natural neutrinos from the sun. But the only neutrinos we've observed from an identified extra solar location, the 1987A http://en.wikipedia.org/wiki/SN_1987A [wikipedia.org], which was a very close supernova (so close it could be seen with the naked eye). In fact, in that case, the neutrinos arrived before we saw the light. That's not at all connected to the erroneous claim from a few years ago that neutrinos were going faster than light speed. What is happening here is that most of the light in a supernova is formed in the core, and the core of a star is very dense. So it takes a long time for the light to reach the surface of the star. But from the standpoint of neutrinos even very dense star isn't that much of an issue so they can get to the surface much faster. It is possible that this sort of work will give us better understanding both such neutrinos and what to expect when we do observe them from other close supernova.

Neutrinos are still a major area where there's a lot we don't understand, and this research is going to possibly have major implications for our understanding of these elusive particles.

Still, I think in this century materials and systems will be developed which can detect neutrinos 1000 times better than today. That opens up new possibilities in astronomy (solar neutrinos, AGN, looking past the CMB just to name a few).

Yes, neutrinos start the SN explosion according to current models. However, the timing results for SN 1987a were actually inconclusive, as observatories were not connected to calibrated precision timing systems.

Still, I think in this century materials and systems will be developed which can detect neutrinos 1000 times better than today. That opens up new possibilities in astronomy (solar neutrinos, AGN, looking past the CMB just to name a few).

It could be that there is no magic material that improves interaction and that we will have to just deal with the fact that most neutrinos will not hit anything, regardless of how the detector is constructed. The only road forward for improved detection then would be to make a larger detector. Although projects like IceCube [wikipedia.org] already have detector volumes over a cubic kilometer.

Um, how? Using Dark Matter detectors? Look, neutrinos couple to existing, known particles -- leptons, although via other more complex processes e.g. inverse beta decay they can couple to protons in nuclei as well. The response we can detect is in proportion to the density of those particles in a medium that can function as a detector, which in turn is proportional to straight up mass density. The interaction probability is phenomenally low for all of the known particles, so one requires large volumes of material to make ANY kind of detector. There are strict limits on the density of ordinary matter, and even more stringent limits on the density of matter that can conceivably used as a detector

So I'm curious -- given that the ratio between the mass density of water and the mass density of e.g. Tungsten, Uranium, Plutonium is only a single order of magnitude, and a reaction cross section that AFAIK depends solely on the mass density, where exactly are the other two orders of magnitude going to come from from any possible variation in materials?

"Systems", well, perhaps. If we use underground cavities filled with water to look for Cerenkov radiation, or chlorine detectors to look for outgassing Argon, we can always make them 10x bigger and hence increase the detector volume 1000 fold. But this is morally equivalent to building 1000 detectors like we have today and combining the data, and it still leaves us with the same issues if one wishes to determine flavor information and not just raw e.g. neutral current flux. Indeed, to get flavor information we will very likely be limited to building 1000 detectors to get 1000 fold increase in data simply because there are size constraints on detectors that are going to detect and resolve e.g. a muon produced in a charged current interaction.

So sure, we can always scale up our existing technology or minor improvements of it to improve detection rates. But I don't think that materials or systems that improve the scaling of the detection technologies we already have are particularly plausible, based on what I now know. So what did you have in mind (as in, do you know something I don't)? Are there other materials that are likely to have orders of magnitude higher cross section for inverse beta decay, or are you just thinking of building bigger/more detectors?

Not practical anytime soon, but maybe someday, we could create materials with muons in place of electrons. Effectively much smaller atoms. Much denser, many more quarks per cubic cm to catch neutrinos. How to keep it stable instead of being ridiculously radioactive and going poof in a microsecond, and make bulk amounts, I have no idea. Crazy new things will be invented in the coming decades. We can't discount the possibility of some impossible-today new material that can catch neutrinos.

Muonic atoms have been studied for some time now, and there is no indication of any special way to get stability, neither in theory or in experiment. It would have great potential for use in muon-catalyzed fusion if there was some secret to even marginally longer lifetimes. This is not to say it is impossible, as current theories could be wrong. But expecting that will be a particular likelihood has less motivation than expecting we'll fix the long list of problems with the Alcubierre drive. There is al

Doesn't the problem of the muon sticking to the resulting helium nucleus too often pose a more important problem for muon catalyzed fusion than the muon life-time? Would the impossible increase in muon life-time suggested here help muon-catalyzed fusion at all?

Early work and theory suggested that even with a 100% stable muon, the muon sticking would be too much to make it worth while. Although later experiments gave more optimistic values for the sticking percentage, but still not quite marginally viable even if assuming the rest of the process was 100% efficient. Longer lived muons however would give a lot more options for generating them and could lead to greatly increased efficiency in other parts of the system. There are also several reactions and possibil

Can any useful deflection (lensing) of neutrinos be accomplished with anything less than a black hole?I came across articles about stars and the CMB as lensing possibilities.I am wondering if the Earth itself or a lead lens could have any effect at all.

Unfortunate timing considering what thread your comment follows. Although more unfortunate that you added to the mess by thinking your complaints about Slashdot were more important or interesting than the rather nerdy story. So, you're a contributing part of the very problem you complain about.

It would have been easy enough to just link say to the Wikipedia article on neutrino oscillation [wikipedia.org] which discusses other examples of experimental measurements of neutrino oscillations. You could have been more techni

From the experiments I've seen neutrinos travel at the speed of light, which was found and reported with great relish by physicists wishing to undermine the faster than speed of light experiments from a few years ago.

The problem with this is that while it solves one issue, it opens up another, because only objects without mass are supposed to be able to travel at the speed of light such as photons, but neutrinos have been found to change "flavors" or types. And for them to do so requires that they have mass

Neutrinos haven't been found to be traveling at the exact speed of light, only at speeds indistinguishable from the speed of light. The error bars around such measurements are not able to distinguish between a particle traveling exactly at the speed of light and a very light particle with a lot of energy to make it travel almost at that speed.

Lets take the neutrino to have a mass near the upper bound of current estimates of 2 eV, and use for example the neutrinos from the 1987A supernova which had energy o

This explanation is insufficient. If neutrinos were indeed massive particles we'd see a wide distribution of their velocities, just like we can observe slow and fast protons, slow and fast electrons, slow and fast everything that moves slower than c. Yet, in 100% of experiments that have been done all neutrinos are propagating through space at the speed close to or exactly equal to c.

The reason for this is the extremely small mass of neutrinoes. The current experimentally-derived upper bounds on their mass is around 1 eV (in contrast, an electron has a mass of 511,000 eV). This means that any process that creates a neutrino will give it enough energy to send it off at ultrarelativistic speeds. Even something simple like neutron decay can impart 1 MeV of kinetic energy to a neutrino, which, as the grandparent calculates, means the neutrino is traveling at 0.999999999999*c. Only chemical reactions would release a small enough amount of energy to have non-relatvistic neutrinos. But, chemical reactions don't release neutrinos.

This is why we only see speed-of-light neutrinos. This is also why it's taken so long to discover that they have mass.

This explanation is insufficient. If neutrinos were indeed massive particles we'd see a wide distribution of their velocities, just like we can observe slow and fast protons, slow and fast electrons, slow and fast everything that moves slower than c.

That's completely mistaken. We don't find a wide distribution of neutrino velocities because it takes very little energy to make a neutrino go very close to the speed of light (this happens because neutrinos have *very* little mass). This means that there's a very small probability for a neutrino to have a small velocity relative to anything else -- you just have to sneeze at it (that is, interact with it in almost any way) to send it flying away at close the speed of light. So it's *expected* that you'll never actually see a slow neutrino.

There is already too much evidence against Relativity Theory as it presently is.

That's just bullshit. It's true that General Relativity doesn't fit at all with Quantum Mechanics, but there's *no* compelling evidence at all against Relativity (either General or Special). There's *no* known experiment that gives a conclusive result that's different from what Relativity predicts. People are working on other theories because General Relativity doesn't fit with Quantum Mechanics, not because there's evidence against Relativity.

It's true that General Relativity doesn't fit at all with Quantum Mechanics

That is overstating things a bit. For the most part, GR and things like QFT get along just fine, and it helps that space-time in GR is locally pretty flat most of the time. And some work comes from just using them together anyways. There are some obvious conflicts (although some I think are more for idealogical reasons than experimental reasons), but it comes down to specific situations, which is part of the difficulty of testing proposed replacements.

My thanks to you and the other AC's who've left me with a far better almost-understanding of neutrinos than when I started. This is the kind of discussion I relish here. It informs, teases (possibles, speculation, what if, how the hell?), and delights my inner scientist. Now I'm off to refresh my non-understanding of CP-violation, causality, and timeline. Again, thank you.